Niobium has been used as an additive to steel because niobium is effective in stabilizing carbon in steel and prevents the progression of corrosion among particles. A niobium alloy has been used as a material of a conductive tube attached to the light emitting portion of a high-pressure sodium lamp, a superconductive material and an additive to a super alloy. Recently, demand for niobium oxide is notable because niobium has been widely used in electronic and optical fields. Particularly in those fields, highly pure niobium is indispensable. Purification of niobium compounds has been achieved by various methods depending on the niobium compounds serving as starting materials. For example, the purification of niobium oxide has been achieved by differential crystallization, solvent extraction, ion-exchange resin-based separation, distillation, etc. However, ores used for the extraction of niobium such as columbite or niocalite contain tantalum together with niobium. Moreover, since niobium shares many physical and chemical properties with tantalum, it has been extremely difficult to separate niobium from tantalum. Accordingly, many methods proposed heretofore include extracting metallurgical products containing the two elements from the ores.
For example, the ore containing both niobium and tantalum is ground to a powder which is treated with an acid mixture comprising hydrofluoric acid and mineral acid, e.g., sulfuric acid, so that niobium and tantalum are dissolved in the acid mixture together with other metal impurities such as iron, manganese, calcium, rare earth elements, etc. The solution is allowed to contact with an organic solvent such as ketone, ester or ether of a lower fatty acid, particularly methyl-isobutyl-ketone, and niobium and tantalum are extracted through the organic phase. According to a second method disclosed in A published Japanese Patent Application, Publication No. S58(1983)-176128, the solution described above is allowed to pass over an F-type anion-exchange resin layer to allow niobium and tantalum to be adsorbed to the resin layer, thereby separating the two elements from other metal impurities. Then, the two elements are dissolved in aqueous solutions of hydrofluoric acid and ammonium chloride to be recovered later.
Various processes for separating niobium from tantalum have been investigated. One such conventional process consists of separating the two elements dissolved in an aqueous solution based on the difference in their concentrations of salting-out and hydrogen ions. Specifically, when the concentrations of hydrofluoric acid and mineral acid constituting an acid mixture dissolving niobium and tantalum compounds are reduced, NbF72− is converted into NbOF52− while TaF72− stays unchanged, and thus it is possible to selectively extract tantalum using an organic solvent such as methyl-isobutyl-ketone. Alternatively, it is also possible by a process contrary to the above to extract tantalum and niobium using an organic solvent and then to selectively extract niobium using an acidic aqueous solution. However, as discussed in A published Japanese Patent Application, Publication No. S62(1987)-158118, the process for obtaining a highly pure niobium or tantalum using a ketone such as methyl-isobutylketone (MIBK) becomes a publicly known technique. Yet another method is disclosed in A published Japanese Patent Application Publication No. S64(1989)-31937. The method includes the use of a porous support made of active carbon or polypropylene to which an oxygen-containing organic solvent such as trioctyiphosphine oxide or an organic solvent chosen from alkylamines in which the alkyl group has four or more carbon atoms is attached or linked, and comprises the step of flowing an acidic aqueous solution containing the two elements over the support so that only tantalum is adsorbed to the support and separated from the acidic solution. However, it is difficult to obtain a support to which an organic solvent is linked. Alternatively, if the organic solvent is impregnated into a porous support, dissolution of the organic solvent to the water phase poses a problem. However, the above extraction method greatly complicates the process, for example, it requires multiple extraction steps: the water phase must be washed with an organic solvent in a series of mixing-settling steps.
In view of these problems inherent to the conventional methods, an object of the present invention is to provide a method for purifying a highly pure niobium compound and/or tantalum compound in a more simplified manner at a lower cost than is possible with the conventional methods.